title: Calpain-dependent cytoskeletal rearrangement exploited for anthrax toxin endocytosis.
abstract: The protective antigen component of Bacillus anthracis toxins can interact with at least three distinct proteins on the host cell surface, capillary morphogenesis gene 2 (CMG2), tumor endothelial marker 8, and β1-integrin, and, with the assistance of other host proteins, enters targeted cells by receptor-mediated endocytosis. Using an antisense-based phenotypic screen, we discovered the role of calpains in this process. We show that functions of a ubiquitous Ca(2+)-dependent cysteine protease, calpain-2, and of the calpain substrate talin-1 are exploited for association of anthrax toxin and its principal receptor, CMG2, with higher-order actin filaments and consequently for toxin entry into host cells. Down-regulated expression of calpain-2 or talin-1, or pharmacological interference with calpain action, did not affect toxin binding but reduced endocytosis and increased the survival of cells exposed to anthrax lethal toxin. Adventitious expression of wild-type talin-1 promoted toxin endocytosis and lethality, whereas expression of a talin-1 mutant (L432G) that is insensitive to calpain cleavage did not. Disruption of talin-1, which links integrin-containing focal adhesion complexes to the actin cytoskeleton, facilitated association of toxin bound to its principal cell-surface receptor, CMG2, with higher-order actin filaments undergoing dynamic disassembly and reassembly during endocytosis. Our results reveal a mechanism by which a bacterial toxin uses constitutively occurring calpain-mediated cytoskeletal rearrangement for internalization.
authors.affiliation: Departments of Genetics and Medicine, Stanford University School of Medicine, Stanford, CA 94305.
authors.forename: Sun-Young
authors.initials: SY
authors.lastname: Jeong
authors.forename: Mikhail
authors.initials: M
authors.lastname: Martchenko
authors.forename: Stanley N
authors.initials: SN
authors.lastname: Cohen
mesh_terms: D019943:Amino Acid Substitution; D000818:Animals; D000942:Antigens, Bacterial / Q000235:genetics / Q000378:metabolism*; D001408:Bacillus anthracis / Q000378:metabolism* / Q000472:pathogenicity; D001427:Bacterial Toxins / Q000235:genetics / Q000378:metabolism*; D002154:Calpain / Q000096:biosynthesis* / Q000235:genetics; D002460:Cell Line; D003599:Cytoskeleton / Q000235:genetics / Q000378:metabolism* / Q000473:pathology; D015536:Down-Regulation / Q000235:genetics; D004705:Endocytosis; D022001:Focal Adhesions / Q000235:genetics / Q000378:metabolism / Q000473:pathology; D005786:Gene Expression Regulation; D051379:Mice; D020125:Mutation, Missense; D021381:Protein Transport / Q000235:genetics; D018000:Receptors, Peptide / Q000819:agonists / Q000235:genetics / Q000378:metabolism; D016608:Talin / Q000235:genetics / Q000378:metabolism
chemical_list: D000942:Antigens, Bacterial; C523013:Antxr2 protein, mouse; D001427:Bacterial Toxins; D018000:Receptors, Peptide; D016608:Talin; C030325:anthrax toxin; C496077:Tln1 protein, mouse; D002154:Calpain; C570925:Capn2 protein, mouse
keywords: MDL28170; calpastatin; lethal factor